There is a long-felt need for a cementitious material having sufficient strength while also possessing favorable toughness. Typically, overall strength decreases, with one example being compressive strength, while toughness increases. Strength is defined as force per unit area, i.e. energy per unit volume, while toughness is energy per unit area.
The toughness index, as evaluated by the area under the load-deflection curve, is considered as a measure of the energy absorption capability of the material during fracture in the non-linear portion of the curve. The ACI Committee 544 (83-85) has defined the toughness index as the measure of the amount of energy required to deflect a fiber concrete beam, used in the modulus of rupture test, by a given amount compared to the energy required to bring the fiber beam to the point of first crack.
In ductile materials, with one example being metals, the fracture process zone, though small, is surrounded by a large nonlinear plastic zone, whereas in quasi-brittle materials, with an example being concrete, the fracture process zone occupies practically the entire zone of nonlinear deformation. In contrast, the nonlinear zone is practically absent in brittle materials.
When creating cementitious materials for ballistic or blast applications, two very important design considerations are meeting the specified mechanical strength requirements and minimizing the amount of spall at failure. One traditional means for designing hydraulic cement based materials to meet such performance requirements is to densify the material's microstructure while incorporating some means of reinforcement. Commonly used reinforcing materials are often fibrous materials—likely steel fibers. Steel fibers are primarily responsible for increases in ductility for cementitious materials possessing dense microstructures. In most densified cementitious materials, the coarse aggregate in mix designs is substituted with finely ground fillers in conjunction with high performing water reducers to create a very dense microstructure possessing fewer capillary networks ultimately leading to a somewhat ideal “spacing packing” design for much better distribution of applied load. This methodology is also referred to as the “packing density optimization principle” and such materials may also be referred to as “densified spacing packing” (DSP) materials. A few common examples of finely ground fillers are ground quartz, silica fume, fly ash and ground granulated blast furnace slag. Typically these fine materials are of smaller particle size when compared with un-hydrated cement grains allowing the finely ground fillers to occupy spaces between cement grains providing homogeneity throughout the microstructure for better, more consistent distribution of applied loads. These dense materials often possess incredible compressive strengths, though the tensile strength often remains in the realm of 10% of compressive strength. As previously mentioned, fiber addition often improves tensile strength. Materials falling into this dense microstructure classification with incredibly high compressive strength values are often referred to as ultra high performing concrete (UHPC) or very high performing concrete (VHPC). The densified spacing packing “DSP” based cementitious materials are however distinctly brittle, the more so the higher the compressive strength. In fact, although the tensile strength is about one tenth of the compressive strength, the material is intrinsically brittle. Therefore, a process such as is described herein can be incorporated into mix designs for DSP based materials for mitigating spall in blast or ballistic applications.
A material such as is described herein embodies a completely different approach for meeting the specified mechanical strength requirements for ballistic or blast applications while minimizing the amount of spall at failure. A material such as is described herein includes latex polymers in cementitious materials mix designs. As opposed to the “densification of microstructure” methodology, the microstructures of polymer modified cementitious materials become less dense with increases in polymer content due to tough, flexible polymer films occupying the void spaces within the microstructure. The microstructural behavior of polymer modified materials changes with increases in polymer concentration. Low polymer concentrations create very tough materials which fail in somewhat brittle fashion; whereas, high polymer concentrations create somewhat flexible materials which fail in elastic fashion. Essentially, polymer concentration influences the material's modulus of elasticity. Higher concentrations of polymer per specific volume of microstructure decrease the modulus of elasticity thereby forming a more ductile material.
These polymers are film forming thermoplastic materials widely known for use as mechanical property modifiers for specialty cementitious products. These film forming polymers are known to increase direct tensile strength of mortars by filling void spaces with tough, flexible polymer film. These film-forming polymers are known to increase adhesion strength to materials by forming mechanical bonds with the substrate. For example, polymer modified mortars with higher polymer/cement ratio demonstrate acceptable adhesion performance to non-porous substrates, with an example being glass. Such improvements in both adhesion characteristics and ductile behavior highlight a material such as is described herein as ideal for use in applications requiring coating of other construction materials for mitigating spall during failure of said materials.
In contrast to the mechanical property correlations inherent to traditional cementitious materials, polymer modified cementitious materials possess significantly higher direct tensile strengths and flexural strengths (modulus of rupture). In polymer modified cementitious materials, both direct tensile strength and flexural strength increase to some optimum value before beginning a decreasing trend with increases in polymer/cement ratio. Compressive strengths of polymer modified cementitious materials are often lower than compressive strength of traditional cementitious materials due to the presence of tough, flexible polymer film within the pore network of the polymer modified cementitious materials. The polymer alone behaves as a rubber like material which correlates to decreases in compressive strength with increases in polymer dosage. Such behavior of polymeric material has the potential to lead to creep type behavior for select mix designs. Depending upon area of application, mitigating creep behavior should not be limited to methods such as selecting interlocking aggregates or adding mesh type reinforcing materials, not limited to mesh materials containing fabrics, yarns, wires or nano-tubes.
The mechanical property performance of polymer modified cementitious materials can be strictly controlled by varying the polymer/cement ratio. In other words, the ductile behavior of polymer modified cementitious materials can be strictly controlled by adjusting the polymer/cement ratio. The mechanical property performance of polymer modified cementitious materials can also be influenced by both polymer chemistry and polymer glass transition temperature (Tg); however, the polymer/cement ratio is always to be taken into account when making a material such as is described herein.
In latex polymer modified cementitious materials, the polymer is dispersed somewhat uniformly throughout the material microstructure. Increasing the polymer/cement ratio increases the amount of polymer per given volume of material. The polymer is a tough, flexible material. As the polymer/cement ratio increases, the amount of tough, flexible material increases allowing a transition from brittle to ductile behavior with increasing polymer/cement ratio. As previously mentioned, both direct tensile strength and flexural strength increase up to an optimum value before beginning a gradual decrease with increasing polymer/cement ratio. Even though compressive strength, tensile strength and flexural strength begin to decrease with increasing polymer/cement ratio, all is not lost in terms of material behavior as the percent elongation at break for uniaxial direct tensile testing continually increases with increases in polymer/hydraulic binding agent ratio excluding influence of other reinforcing materials with examples not being limited to fibers, rebar and mesh. Such behavior highlights a material such as is described herein as ideal for mitigating spall in specific applications.
High polymer/cement ratio cementitious materials exhibit extraordinary behavior when compared with traditional, brittle cementitious materials. High polymer/cement ratio cementitious materials will display a yield curve on a stress/strain diagram likely with greater area under the curve when compared with traditional cementitious materials. High polymer/cement ratio materials often display a longer plateau for the yield curve as the material fails and the platens of the testing machine move a greater distance during failure when compared with materials more brittle in nature. Such a longer plateau and greater area under the stress/strain curve highlight the increased toughness of these high polymer/cement ratio materials. Aforementioned behavior, akin to that of greater elongation at break during uni-axial direct tensile strength testing of a material such as is described herein, creates a set of circumstances such that a material such as is described herein can incorporate various materials, which previous reports create the common perception of it being well known said materials are less effective when incorporated with cementitious materials versus incorporation with materials more elastic in behavior. An example of such material should not be limited to aramid yarn spun in the form of Kevlar fabric. It is well known to those skilled in the art that Kevlar K29 mesh is commonly used as the workhorse in soft armor or soft ballistic materials. Kevlar K29 is ideal for use in soft armor as K29 may elongate some 3% or more before failure. When constrained, it is believed a certain number of layers of K29 are less effective when compared with an identical number of layers of K29 allowed room for elongation when subject to loading. The increased ductile behavior of higher polymer/cement ratio cementitious materials creates an environment conducive for incorporation of high performance, engineered type materials into design of materials resulting from a process such as is described herein. Additionally, a material such as is described herein may serve as a suitable material for incorporation of recycled materials, not being limited to recycled aramid type materials.
When subjected to sudden high velocity impact or blast loading, the amount of generated spall decreases significantly with increasing polymer/cement ratio. The continuous polymer film throughout the material microstructure binds the constituent materials together. The tough, flexible polymer film bridges micro-cracks as they form, thus increasing the material's capacity for energy absorption. This adequately explains the observed results for reduced cracking and spall for the tested materials with increases in polymer/cement ratio. A material such as is described herein is seen as a suitable low cost alternative material for inclusion during manufacture of Chobham type armor components, or any armor type components, whether they be used for construction of personal defense devices or armoring of vehicles, components, structures, vessels, crafts or other tangible objects.
Given the ability of a material such as is described herein to absorb more energy during crack formation processes, such microstructural behavior creates ideal circumstances for incorporation of specifically designed objects, with examples not being limited to items comprising wood, glass, ballistic glass, plexi-glass, safety glass, thermoset polymer based materials, thermoplastic polymer based materials, composites, polymer composites, ceramic type materials, ceramic tiles, tiles produced from any material or combination or materials, ball bearings, metals, alloys, boron carbide, silicon carbide, aluminum oxide, aluminum nitride, titanium boride, or the like for purposes of either absorbing energy of projectiles or altering the path of the projectile, thereby increasing the total distance the projectile must travel before nearing a surface and creating spall as the projectile progresses some distance through the microstructure of the host material. Behavior of such materials is deemed as ideal for meeting the need for cheaper construction materials for the purpose of providing a specified degree of protection for persons or property subject to ballistic or blast loading. Such construction materials can be used to form virtually any conceivable object or geometry, either load bearing or non-load bearing, either as a sole material or in combination with other construction materials and methods of construction.
Various methods exist for classifying behavior of materials subject to blast or ballistic type loading. Common test methods or schemes for ballistic type loading should not be limited to Underwriters Laboratories UL 752, National Institute of Justice NIJ 018.01, United States State Department SD-SDT-02.01, ASTM F-1233, European Normal Standard DIN EN 1063 or HP White Laboratories HPW-TP 0500.02. A listing of requirements set forth when testing according to UL 752 is discussed in reference with Table 9 of this document. Examples of a material such as is described herein tested according to UL752 were tested at a distance of approximately 10 ft or approximately 3.05 m from the sample.
Numerous reports exist describing a need for development of higher performing cementitious materials which will maintain greater degrees of structural integrity with reduced potential for releasing fragmented projectiles when exposed to either blast or ballistic type forces. A material such as is described herein accordingly provides higher-performing cementitious materials which will maintain a greater degrees of structural integrity with reduced potential for releasing fragmented projectiles when exposed to either blast or ballistic type forces.